This invention relates generally to arrays of image detectors and more particularly to the use of a dithering device to correct for differences in the responses of the individual image detectors forming the array.
An array of image detectors is typically composed of a large number of individual semiconductor detectors. Each of these detectors generates an electrical response upon exposure to electromagnetic radiation, such as infrared radiation. However, because of manufacturing constraints and environmental conditions, the detectors fail to have identical operating characteristics. That is, substantially similar levels of infrared radiation at two different detectors can generate different responses at each of the detectors.
To the extent that two detectors generate different electrical responses upon exposure to the same level of infrared radiation, we say that the detectors display "spatial non-uniformity". Spatial non-uniformity between detectors is caused by fixed pattern noise that includes individual detector offsets, residual gain error in the detectors, fixed pattern electronic noise, and non-dithered optical structure in the detectors field of view. Pixel offset errors in a detector can be modeled by adding a fixed DC value to each detector's ideal response, and pixel gain errors in a detector can be modeled by scaling each detector's ideal response.
One approach for correcting fixed pattern noise errors employs factory calibration of the detector array. Factory calibration involves exposing the array to a uniform source and tabulating the response of each detector in the array. The tabulated entries consist of gain and offset corrections for each detector in the array. The entries in the table can be applied against corresponding detectors to generate a corrected image. The factory calibration solution, however, suffers from multiple drawbacks. First, the pixel offset errors may not be linearly dependent, rather they may have non-linear temperature variations. Thus, factory calibration must take place over a broad range of temperatures to perform effectively. Second, this solution cannot correct for short-term temporal variations in pixel offset error that occur during operation of the array. For instance, variations in temperature of the detector array can create significant offset variations over time. Finally, this method requires recalibration to correct for long-term unpredictable changes in pixel offset errors that occur as the array components age.
An alternative approach eliminates the disadvantages associated with factory calibration by calibrating the focal plane array while it is in use. This is done by placing a rotating plate in front of the detector array, such that the array is alternately exposed to the image under observation and to a signal of known intensity. The fixed pattern noise is removed by subtracting a detector's response to the known signal from the detector's response to the observed image.
This solution has two drawbacks. First, by requiring a means for alternately exposing the array to the observed image and to a signal of known intensity, this solution requires additional complex mechanical or optical elements. Second, by requiring that the focal plane array spend time viewing a signal of known intensity instead of the scene under observation, this solution inevitably degrades the array's ability to track fast moving objects and reduces the potential signal to noise ratio of the sensor output.
O'Neil, in U.S. Pat. No. 5,514,865, discloses another approach for correcting spatial non-uniformities in a detector array. The O'Neil system employs a dithering system that spatially dithers the observed image across the detector array to correct the gain and offset errors in the array of detectors. The detector array line of sight is moved between consecutive image frames according to a predetermined pattern. This dithering of the array's line of sight causes different detectors to image the same location in the scene during different image frames, and causes two adjacent detectors to scan between the same two points in the scene during a cycle of the predetermined dither pattern. Theoretically, if two ideal detectors view the same part of an image then the two ideal detectors generate the same response to that part of the image. Differences existing in the response of two detectors viewing the same part of an image can accordingly be characterized as error in the detector response.
The O'Neil system calculates the detector error separately as a gain error and an offset error. The gain error is calculated by analyzing variations in gain from detector to detector in the array. During the dither pattern, two or more detectors will traverse the same path between the same two points in the scene. The difference in intensity between these two points is referred to as the scan gradient. Gain corrections are determined from the difference in the scan gradients of these two common points of the scene scanned by different detectors.
The offset error is calculated from variations in offset from detector to detector in the array. During the tracing of the dither pattern, a plurality of detectors will image the same point of the scene. The intensity sensed by these detectors should be the same. Thus, offset corrections are determined by measuring the difference in intensity of a point sensed by two or more different detectors.
The O'Neil system is understood to use average signal intensities to determine detector offset values, and to use average signal gradients to determine gain values. This system, however, appears to have difficulty with low frequency variations in offsets across the detector array. In addition, the O'Neil technique is very sensitive to the accuracy of the spatial movement of the dithering mechanism. This method also encounters difficulties when one or more of the detectors in a dithering cycle is so defective that it generates a signal of dubious reliability. The defective detector, when incorporated into the average detector response, can skew the average detector response of those detectors lying in the dithering cycle.
Accordingly, one object of the present invention is to provide an apparatus and method for correcting fixed pattern noise errors, both globally and locally, in a planar array of detectors.
This and other objects will be apparent from the following description.